One cold diver turns over chunk of glacier (7)
I believe the answer is:
iceberg
'chunk of glacier' is the definition.
'one cold diver turns over' is the wordplay.
'one' becomes 'I' (Roman numeral).
'cold' becomes 'c' (eg on taps).
'diver' becomes 'grebe' (diving bird).
'turns over' says the letters should be written in reverse.
'grebe' in reverse letter order is 'eberg'.
'i'+'c'+'eberg'='ICEBERG'
(Other definitions for iceberg that I've seen before include "unemotional person" , "One sank the Titanic" , "Hazard of polar seas" , "Titanic sinker - lettuce" , "Broken piece of a glacier" .)